My hope was that making it a child layer it would speed up the render time, though I can see from the announcement thread why that might not be so.
Having managed to make it a child layer the render does seem to run more quickly but maybe that's an illusion caused by too much impatience
The erosion code is being used in this case, on stdout I can see messages about the cache size.
And I've just seen Chris's comment (in the announce thread) about using the shader as a mask which looks to be what I should really be doing